How do I fallback to boot from local hard drive in iPXE script?

The normal way to do that is just to setup your local drive as the next item in the BIOS boot order and use the exit command in the iPXE shell or script.

To answer my own question.

sanboot --no-describe --drive 0x80

or it might be 0x81 or 0x82

I was able to boot a linux USB drive on 0x82.